In accelerated efforts to isolate Iran, the Obama administration has given hint to a change in America's ballistic missile defense system plans. President Obama told reporters on Tuesday that in a letter to Russia, he said that lessening Irans commitment to nuclear weaponsreduces the pressure for, or need for, a missile-defense system. Although world leaders deny a quid pro quo, the implicit deal would seem to guarantee assistance in dealing with Iranian nuclear weapons ambitions in exchange for a cessation of US plans to build interceptor missiles silos in Poland and a tracking radar in the Czech Republic. Last summer, we looked at how the evolving U.S. plans aimed at rogue regimes were fueling tensions with Russia.

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The U.S. ballistic missile defense shield has been up and running since 2004, and it's growing. If an intercontinental ballistic missile (ICBM) were fired at the United States, there would be as many as 24 ground-based interceptors (GBIs) ready to fire at it from Fort Greely in Alaska and Vandenburg Air Force Base in California. One or more of these 60-ft.-long, three-stage missiles could be boosted into space, guided by an array of space and terrestrial-based radar systems directed toward the incoming threat. For each GBI's payload--a 140-pound, remote-operated spacecraft called a kill vehicle--this would be a suicide mission. If everything went according to plan, the kill vehicle's four onboard thrusters would slide it directly into the path of the enemy missile and--relying on nothing more than its mass and 20,000-plus mph velocity--the drone would pulverize its target into orbital debris.

Twenty silos that will house GBIs are currently under construction in Alaska, and another 10 are planned for installation in Poland. When completed and equipped, the GBI arsenal will total 54 missiles. In reality, though, ballistic missile defense is far from battle-proven. GBI test shots have included a number of failed interceptions. In this game, failure can be defined as anything less than a perfect record, since it only takes a single warhead slipping through the shield to produce a cataclysmic result.

Geography is a key to understanding how the modern incarnation of missile defense is influencing international politics. Until very recently, North Korea has been the focal point of ballistic missile defense, prompting President George W. Bush in 2002 to mandate a fast rollout of a system, no matter how crude. MDA supporters applauded that move in 2006, when North Korea tested a Taepodong-2 missile, which according to the Missile Defense Agency (MDA) has enough range to reach San Francisco and other West Coast targets.

According to MDA's director, Lt. Gen. Henry Obering, According to MDA's director, Lt. Gen. Henry Obering, having a ballistic missile defense system at that time provided an alternative to a preemptive strike on North Korea's missile fields, "which could have escalated the situation and been incredibly destabilizing to the region," he tells PM. The Taepodong-2 failed less than a minute after launching, but to Obering the GBI system had already done its job by simply existing--without any defense, the situation could have lead to an attack on North Korea long before the missile had a chance to harmlessly sputter out. As for the GBI's testing track record, Obering points out that of the last nine long-range tests, six have been successful. The tests are also getting more challenging, with four of those successful interceptions involving targets using countermeasures, such as the release of a decoy balloon designed to steer the interceptor away form the warhead. "Today, we are well beyond the question of `Does this work?' " Obering says. "There's no doubt in my mind."

From the base's location 100 miles southeast of Fairbanks, missiles from Fort Greely can theoretically intercept threats to both coasts. But its missiles, and the handful at Vandenburg, are particularly well-suited to handle a strike crossing the Pacific. The geometry of an Iranian attack on the East Coast would mean a smaller window for interception. So as Iran's nuclear ambitions increased, the decision was made to set up another defensive front.

That brings the missile defense debate to our allies in Eastern Europe, which are in range of Iranian missiles. Poland has already agreed to an installation of 10 GBIs, and earlier this week the Czech Republic signed a deal to accept a massive radar system, which would help guide those interceptors. For the most part, the new batch of GBIs will be almost the same as the existing missiles, with two stages instead of three (due to the close proximity to Iran, the kill vehicles need to be activated earlier) but nearly identical silos.

The stated goal of the Polish system is to thwart Iran, but Russia is not thrilled with the presence of U.S.-run silos along its border. Officials there have said that the interceptor tubes could be loaded with offensive ordnance, such as nuclear warheads. The Moscow-based Interfax news agency has reported that, because of the recent missile shield deal, a Russian general has threatened Poland with the possibility of a nuclear strike.

The U.S. has tried to mollify Russian concerns. Obering says that a number of transparency measures and other options have been presented to Moscow. The United States has also offered to ready the missile field, then place all of the GBIs into storage. The clamshell doors capping each silo could then be opened, revealing the empty tubes to satellite and aerial cameras. Reloading the field, says Obering, would take "a matter of days, not months," provided an Iranian threat starts to emerge.

Moscow had not yet responded to these specific overtures, but the MDA has even gone so far as to point out the inadequacies of its GBIs. They aren't fast enough, says Obering, to catch up with an ICBM fired from Russia at the United States. And even if they were, the 54 proposed interceptors spread out through Europe, Alaska and California wouldn't stand a chance against hundreds of Russian ICBMs. It's not hard to see the irony here, that the failure of Reagan's Star Wars vision, of a missile shield capable of fending off a swarm of Soviet warheads, has become a diplomatic ploy to head off renewed nuclear tensions with Russia.

Ground-based systems are only one layer of MDA's proposed network--and with new technology comes new geopolitical ramifications. The agency is hoping to field systems that could stop a ballistic missile before or after launch. The much-talked-about Airborne Laser, a 747 armed with a chemical laser, is scheduled to intercept a target missile during a test before it reaches orbit in 2009, and could be deployed as early as 2015. There are also missile batteries, such as the improved versions of the Patriot anti-aircraft system, that could serve as a last-ditch backup, targeting an incoming warhead during the seconds between reentry and impact.

To counter the possibility of increasingly sophisticated threats, such as Russian-designed missiles that can split up into a constellation of real or decoy warheads, every GBI will eventually be upgraded with a "multiple kill vehicle," which would have its own cluster of ramming spacecraft. And down the line, in an echo of proposed Star Wars programs of old, Obering expects the MDA to focus on space and lasers. Specifically, he believes that space-based sensors will provide enhanced tracking capabilities, and that directed energy weapons, such as solid-state lasers, will become more viable for missile interception. This is not the ballistic missile shield that Reagan envisioned; these days, the potential threat is more limited, and success is more easily imagined.